Apparatus and method for determining when the frequency of an alternating signal is below a predetermined threshold

ABSTRACT

A frequency-responsive integrated circuit (IC) for determining when the frequency of a clock pulse input signal is below a predetermined threshold level, the IC including a capacitor charged up at a nearly constant rate by a current source. If the capacitor voltage reaches one-third of the DC power voltage, and input pulses are received, the capacitor is discharged to start another charge-up cycle. If no input pulses were received, the capacitor continues to charge up until its voltage reaches two-thirds of the DC power voltage, at which point an output signal is produced indicating that the input frequency is below the predetermined threshold level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to frequency-responsive circuitry. Moreparticularly, this invention relates to such circuitry for determiningwhen the frequency of an alternating signal, such as a clock signal, isbelow a predetermined level.

2. Description of the Prior Art

Lap top computers can be powered either with internal battery, i.e.,when operating in stand-alone mode with the on-board black-and-white,low-power display device they carry, such as a liquid crystal display(LCD), or can be plugged into line power for operation with an auxiliaryhigher-power-consumption three-color display monitor providing asuperior presentation of data but requiring control circuitry whichconsumes power at a rate which would quickly exhaust an on-boardbattery.

When in battery-powered mode, much of the display control circuitry ofthe computer is not needed for the relatively simpler black-and-whitedisplay, and those portions of the computer circuitry can bepowered-down in that mode in order to conserve battery power. Thecomputer advantageously is provided with automatic means for turning offsuch power-consuming circuitry as is not needed when operating with thebattery-powered black-and-white display.

Some three-color monitor displays may require a clock frequency of fromabout 25 to 40 MHz. System considerations, such as testing, may dictatethe use of much lower clock frequencies when operating on a-c linepower, and a limit of 200 kHz might for example be selected as the lowerthreshold for line-power operation. When operating with battery powerfor the on-board black-and-white display, the clock frequency of thecolor display circuitry is set to be much lower, or even at zero,because of the relative simplicity of the on-board display. Controlmeans responsive to clock frequency thus can be used to determine whenthe computer is being operated on battery power. Following suchdetermination, the control means will automatically turn off theunneeded circuits. To be commercially practical, such control means mustbe highly reliable in operation, yet relatively inexpensive to make.

SUMMARY OF THE INVENTION

In a preferred embodiment of the invention, to be described in detailhereinbelow, there is provided a frequency-responsive integrated circuit(IC) which receives the computer clock signal and produces an outputsignal whenever the clock frequency is below a threshold value such as200 kHz. This circuit basically comprises a capacitor charged at anearly constant rate by a current source, and discharged at selectedtimes when certain conditions are met.

As the capacitor charges up, its voltage is sensed by a first comparatorwhich detects when the voltage reaches a relatively low presettrip-point level such as one-third of the DC supply voltage (V_(DD)). Ifthe computer is operating on line power to activate an auxiliary monitordisplay, its clock frequency will be high (e.g., between 200 kHz and 40MHz). This high frequency is indicated by the receipt of clock pulsesprior to the time the capacitor voltage reaches the low presetcomparator reference level, and the appearance of such clock pulses willbe detected and the capacitor discharged at the comparator trip point tostart another charge-up timing cycle. Since the computer is beingsupplied with external line power for the auxiliary monitor display, thefull complement of display control circuitry will be needed and will bepowered.

If the computer is shifted to battery power, the clock frequency dropsto a very low value, or zero, since the on-board display does not needhigh-frequency operation. Now, as the capacitor charges up, no clockpulses will have been received prior to the time its voltage reaches thetrip point of the first comparator. That is, the low clock frequencyresults in a time interval between clock pulses which is substantiallylonger than the time required for the capacitor voltage to reach thecomparator trip point (which may for example be about 20 μsec). Thus thecapacitor continues to charge up beyond that trip point.

As the capacitor continues to charge up beyond that trip point, a secondcomparator senses its voltage. This comparator detects when the voltagereaches a higher preset voltage (e.g., two-thirds of V_(DD)) without anyclock pulses having been received since the start of capacitor charging.This absence of clock pulses indicates a low-frequency clock rate. Upondetecting that condition, the second comparator produces an outputsignal to indicate that the computer is operating on battery power. Thatoutput signal causes power-down of certain components of the displaycontrol circuitry, to conserve battery power.

The frequency-responsive IC described herein incorporates a number ofimportant features to assure that its operation is reliablenotwithstanding uncontrolled conditions such as changes in V_(DD) orprocess variations in manufacturing the IC. Such reliable operation isparticularly important to assure that the IC does not indicate falselythat the clock frequency is below the selected threshold level andswitch the display-control circuitry off. In that regard, features ofthe invention provide a considerable dynamic range of operation withassured performance. Other objects, aspects and advantages of theinvention will be pointed out in, or apparent from, the followingdescription of a preferred embodiment considered together with theaccompanying drawing which shows a circuit diagram of the new integratedcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an integrated circuit embodying thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1, the frequency-responsive IC comprises acharging capacitor C which is an MOS device (MN5) with source and drainconnected to ground for one capacitor terminal, and the gate serving asthe other capacitor terminal. Current is directed to the gate of MN5 bya current source MP3 connected to the positive power line V_(DD). Thecapacitor charging current is compensated for changes in V_(DD) by aband-gap regulator circuit generally indicated at 10, and provided witha start-up circuit generally indicated at 12. Detailed information onband-gap regulators may be found in many prior art publications, such asU.S. Pat. Nos. 3,940,760 and 4,622,512, issued to A. P. Brokaw.

The band-gap circuit 10 includes two bipolar transistors QP2 andQP1/QP3, providing a 2:1 emitter area ratio. The transistor emittercurrents are directed to current mirrors MN1, MN2 and MP1, MP2, whichforce the transistor currents to be equal. With unequal emitter areas, aΔV_(BE) voltage is developed between the emitters. This ΔV_(BE) voltageappears across the resistor R formed by an MOS transistor MN8, anddevelops a corresponding current ΔV_(BE) /R through the bipolartransistors. This current is mirrored to the current source MP3supplying charging current to the MOS capacitor C. Changes in V_(DD)have very little effect on the magnitude of this charging current,because the ΔV_(BE) voltage is essentially proportional only to thevariable of absolute temperature.

This capacitor charging arrangement also provides compensation for theusual variations in the manufacturing process which result indifferences between successive lots of IC chips. For example, if processvariations cause a reduced thickness of oxide beneath the gate of MN5,there would be an increase in the capacitance C tending to decrease therate-of-change of the capacitor voltage ramp-up. However, that samereduced thickness of oxide would occur also for MN8 thereby reducing theresistance R and increasing the charging current into the capacitor.Thus, there is automatic compensation for at least part of the increasein capacitance. In that way, the rate-of-change of the capacitor voltageadvantageously is maintained very nearly constant.

As noted above, the current through resistor R is proportional toΔV_(BE), and thus is proportional to absolute temperature. However, asthe temperature increases, the resistance of R also increases, thustending to maintain the charging current into capacitor C constant inthe face of temperature changes.

After the capacitor C is charged up to a preset level, it is dischargedback to zero volts under certain conditions as described below, and thecapacitor charging cycle begins again. Discharge of the capacitor iseffected by an MOS transistor 14 (MN3) acting as a switch connecteddirectly across the capacitor. This switch is controlled. by a timingcontrol circuit generally indicated at 20 and now to be described.

The timing control circuit 20 includes an input terminal CLK whichreceives the clock signal whose frequency is to be sensed. This CLKterminal is connected to one input of a D-type flip/flop 22 and, throughan inverter 24, to the other flip-flop input to provide a differentialinput signal. The output Q of the flip/flop is fed back through aninverter 26 to the D input, thereby forming a toggle flip-flop. In thatconfiguration, each input clock pulse reverses the state of theflip-flop unless the signal to the reset terminal 28 disables theflip/flop, as will be further described.

When the flip-flop is toggled by a series of input clock pulses, it actsas a 2:1 frequency divider, and produces output pulses having aneffective duty-cycle of 50% regardless of the duty-cycle of the streamof clock pulses. Thus, the constraints of prior art circuits on clockpulse duty-cycle are avoided by this toggle flip-flop arrangement.

If high-frequency clock pulses (e.g., at 40 MHz) are applied to the CLKterminal, and if the flip-flop 22 turned on the switch MN3 to dischargethe capacitor for each clock-pulse cycle, there would be significantcurrent flow through the gate-to-drain capacitance (Cgd) of MN3 as aresult of the considerable voltage activity at the gate. This currentflow could be of the same order of magnitude as the charging currentfrom MP3, and thus could significantly alter the capacitor charging rateand interfere with the desired timing function to be provided by thatcapacitor. The timing control circuit 20, however, is provided withmeans to prevent this adverse result, as will now be described.

Initially, as the capacitor C begins to charge up, the Q output of theflip-flop 22 is forced to logic zero by a signal on its reset terminal28 which also disables the flip-flop. Thus, if a high-frequency (e.g.,40 MHz) stream of clock pulses is applied to the CLK terminal, theflip-flop will (initially) not change state to turn on the switch MN3for each clock pulse received. Thus the capacitor will charge upsteadily and at a fixed rate while a relatively large number ofhigh-frequency clock pulses is received, for example, a hundred or sopulses. This disabling of the flip-flop prevents the voltage swingactivity which otherwise would occur at the gate of MN3 during ramp-upof the capacitor voltage, thereby minimizing the flow of parasitictransient or a-c current into the capacitor C and assuring smooth andsteady ramp-up of its voltage.

As the voltage on the charging capacitor C ramps up, it is sensed by afirst comparator 30 comprising a string of three series-connected MOStransistors MP5, MP7 and MN10,and one additional transistor MP8, withcurrent supplied by V_(DD). These transistors are scaled in such a waythat the comparator trips when the capacitor voltage reaches arelatively low-level reference voltage, e.g., about one-third V_(DD).The resulting comparator output is passed through an inverter 32 to thereset terminal 28 of the flip-flop 22 and enables that flip-flop so thatupon receipt of the next clock pulse the flip-flop switches state andturns on MN3 to discharge the capacitor C.

If the stream of clock pulses on the CLK terminal is at a relatively lowfrequency, such as 10 kHz, the time between pulses will be sufficientlylong that the capacitor C will be able to charge up to beyond the trippoint of the first comparator 30 without being discharged back to zeroby MN3. Ultimately, however, the capacitor voltage will reach the trippoint of a second comparator 40 which comprises a second set of threeseries-connected transistors MP5, MN6 and MN4, and an additionaltransistor MN7. These transistors are so scaled that the comparator trippoint is at a relatively higher voltage than that of the firstcomparator 30, preferably about two-thirds of V_(DD).

When the second comparator trips, its signal passes through an inverter42 to an output terminal OUT. This output signal indicates that thereceived clock pulses are at a frequency below a predetermined threshold(such as 200 kHz). The computer thus is indicated to be on batterypower, and the output signal accordingly effects switchover (byconventional means not shown) to the power-down mode where selectedportions of the computer circuitry are turned off to conserve batterypower.

Although a preferred embodiment of the invention has been disclosedherein in detail, it is to be understood that this is for the purpose ofillustrating the invention, and should not be construed as necessarilylimiting the scope of the invention since it is apparent that manychanges can be made by those skilled in the art while still practicingthe invention claimed herein. For example, the invention may be providedas a cell in an IC chip containing other circuitry, or it may beprovided as a separate chip. Still other variations are readily possibleas is known in the art, including different circuit arrangements forachieving the basic purposes of the invention as set forth in theaccompanying claims.

What is claimed is:
 1. Frequency-responsive means for receiving analternating input signal and producing an output signal indicating whenthe frequency of the input signal is below a predetermined level;saidfrequency-responsive means comprising an integrated circuit formed by aprocess for making a plurality of MOS transistor means with theintegrated circuit including: first MOS transistor means arranged toestablish a capacitor; means for producing a charging current for saidMOS capacitor; said charging current means including second MOStransistor means including at least one MOS transistor serving as aresistor, said charging current means being arranged to produce saidcharging current at a level inversely proportional to the value of saidresistor, control means to receive said alternating input signal andcoupled to said capacitor to control the discharge thereof in accordancewith the frequency of said alternating input signal; said one MOStransistor forming part of a band-gap regulator circuit; said band-gapregulator circuit further including a pair of bipolar transistors drivenwith different current densities so as to produce differing V_(BE)voltages for each; circuit means connecting said one MOS transistor tosaid bipolar transistors to produce a ΔV_(BE) voltage across theresistor defined by said one transistor; and means for producing saidoutput signal when said capacitor is charged by said current to apre-set voltage level.
 2. Apparatus as claimed in claim 1, includingcurrent mirror means connected to receive said current flowing throughsaid one MOS transistor;said current mirror means further including acurrent source to produce said charging current proportional to thecurrent through said one MOS transistor.
 3. Frequency-responsiveintegrated circuit means for receiving an alternating input signal andproducing an output signal indicating when the frequency of the inputsignal is below a predetermined level;said integrated circuit meanscomprising: first MOS transistor means connected to establish acapacitor; means for producing a charging current for said MOScapacitor; means responsive to said alternating input signal and to thevoltage on said capacitor and operable, when said capacitor has beencharged to a first pre-set voltage and signal alternations of said inputsignal are received, to produce a control pulse; switch means having acontrol terminal responsive to said control pulse for discharging saidcapacitor; and means for producing said output signal when saidcapacitor is charged by said current to a second pre-set voltage levelhigher than said first level.
 4. Apparatus as claimed in claim 3,wherein said responsive means comprises frequency-divider meansincluding a toggle flip-flop the output of which is coupled to saidcontrol terminal of said switch means.
 5. Apparatus as claimed in claim4, including first comparator means responsive to the voltage on saidcapacitor and operable to produce an output signal for disabling saidtoggle flip-flop until the capacitor voltage has reached said firstpre-set voltage level.
 6. Apparatus as claimed in claim 5, wherein saidmeans for producing said output signal comprises second comparator meansoperable when said capacitor voltage reaches said second pre-setvoltage.
 7. Frequency-responsive integrated circuit means for receivingan alternating input signal and producing an output signal indicatingwhen the frequency of the input signal is below a predeterminedlevel;said integrated circuit means comprising: first MOS transistormeans connected to establish a capacitor; means for producing a chargingcurrent for said MOS capacitor; means for discharging said capacitorwhen its voltage reaches a first pre-set level and an alternating inputsignal has been received by said means for discharging said capacitor;and means coupled to said capacitor for producing said output signalwhen said capacitor has been charged by said current to a second pre-setvoltage level higher than said first pre-set level.
 8. Apparatus asclaimed in claim 7, wherein said means for discharging said capacitorincludes control means responsive to said alternating input signal andoperable to produce a control pulse;switch means having a controlterminal and operable upon receiving said control pulse to dischargesaid capacitor; and means responsive to the capacitor voltage fordisabling said control means until the capacitor voltage reaches saidfirst pre-set level.
 9. Apparatus as claimed in claim 8, wherein saidcontrol means comprises a toggle flip-flop the output of which iscoupled to said control terminal of said switch means.
 10. Apparatus asclaimed in claim 9, said means for disabling said control meansincluding first comparator means responsive to the voltage on saidcapacitor and operable to produce a signal for disabling said toggleflip-flop until the capacitor voltage has reached said first pre-setvoltage level.
 11. Apparatus as claimed in claim 10, wherein said meansfor producing said output signal comprises second comparator meansoperable when said capacitor voltage reaches said second pre-setvoltage.
 12. The method of monitoring a clock pulse signal to determinewhen its frequency is below a predetermined level, comprising:charging acapacitor formed by MOS transistor means; sensing the capacitor voltageto determine when it reaches a first pre-set level; discharging saidcapacitor after its voltage reaches said first pre-set level and a clockpulse is received; sensing the capacitor voltage to determine when itreaches a second pre-set voltage larger than said first pre-set voltage;and producing an output signal when said second pre-set voltage isreached without a clock pulse having been received.
 13. The method ofclaim 12, wherein the capacitor is discharged by MOS switchmeans;directing said clock pulses to a control circuit for producingcorresponding control pulses; and preventing any control pulse fromreaching said MOS switch means until the capacitor voltage reaches saidfirst pre-set voltage level.
 14. Apparatus as claimed in claim 1,wherein said charging current means includes a source of constantvoltage applied across said MOS resistor to produce a corresponding flowof current therethrough.
 15. Frequency-responsive means for receiving analternating input signal and producing an output signal indicating whenthe frequency of the input signal is below a predetermined level;saidfrequency-responsive means comprising an integrated circuit formed by aprocess for making a plurality of MOS transistor means with theintegrated circuit including: first MOS transistor means arranged toestablish a capacitor; means for producing a charging current for saidMOS capacitor; said charging current means including a band-gapregulating circuit having a pair of bipolar transistors driven withdifferent current densities so as to develop a ΔV_(BE) voltage, saidcharging current means further including a resistor developing saidΔV_(BE) voltage thereacross; current mirror means developing currentproportional to that through said resistor; said current mirror meansfurther comprising a current source tracking the current through saidresistor and serving as the charging current for said capacitor; meansresponsive to said alternating input signal and coupled to saidcapacitor to control the discharge thereof in accordance with thefrequency of said alternating input signal; said responsive meansincluding an MOS switch connected across said MOS capacitor; powersupply means producing a DC power supply voltage; means for producingsaid output signal when said capacitor is charged by the current fromsaid current source to a particular voltage level close to the DC powersupply voltage; said responsive means further comprising a timingcontrol circuit responsive to said alternating input signal and operableto activate said MOS switch under selected conditions to provide for afurther charging cycle; said MOS switch being activated when the voltageon said MOS capacitor reaches a first pre-set level and at least onealternating signal has been received by said means responsive to saidalternating input signal; said timing control circuit including aflip-flop circuit having a reset terminal; and a comparator responsiveto said capacitor voltage and operable to control the signal on saidreset terminal so as to disable said flip-flop until the capacitorvoltage reaches said first pre-set level; said first pre-set level beinglower than said particular voltage level.